Magnetically-focussed cathode-ray tube comprising a tilted and skewed off-axis electron gun

ABSTRACT

The tube comprises an evacuated envelope having a magnetic focus axis and an electron gun spaced from the focus axis and having a gun axis along the general direction of the electron beam in the gun. The electron gun is oriented at an angle with respect to the focus axis, the angle being the sum of a component tilt angle and a component skew angle.

United States Patent Luedicke 1 Jan. 1, 1974 [54] MAGNETICALLY-F()CUSSED 3,708,714 l/1973 Kimura 313/70 R CATHQDERAY TUBE COMPRISING A 2,563,500 8/1951 Snyder, Jr. 313/70 R TILTED AND SKEWED OFF-AXIS ELECTRON GUN Eduard Luedicke, Neshanic, NJ.

Assignee: RCA Corporation, New York, NY.

Filed: Sept. 18, 1972 Appl. No.: 289,780

Inventor:

US. Cl. 313/83, 313/69 C, 313/70 Int. Cl. H01j 29/46 Field of Search 313/69, 69 C, 70,

References Cited UNITED STATES PATENTS Beckers 313/70 R X Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. AIt0rney-Sanf0rd J. Asman, Glenn H. Bruestle and Donald S. Cohen [57] ABSTRACT The tube comprises an evacuated envelope having a magnetic focus axis and an electron gun spaced from the focus axis and having a gun axis along the general direction of the electron beam in the gun. The electron gun is oriented at an angle with respect to the focus axis, the angle being the sum of a component tilt angle and a component skew angle.

7 Claims, 5 Drawing Figures PATENTEDJAX 1 I974 WEE? EMF 2 PATENTEDJAH H974 3.783.326

sum 20? 2' MAGNETICALLY-FOCUSSED .CATHODE-RAY TUBE COMPRISING A TILTED AND SKEWED OFF-AXIS ELECTRON GUN BACKGROUND OF THE INVENTION The present invention relates to cathode-ray tubes in which an off-axis electron beam is deflected over a raster area of a target and is magnetically focussed.

One type of electron tube generally having a deflected and magnetically focussed electron beam is a vidicon camera tube. The tube typically has an elongated cylindrical envelope. Inside one end of the envelope is an electron gun. At the other end of the envelope is a target. Beam deflection means, either internal electrostatic plate pairs or saddle coils located outside the envelope, deflect the beam between the gun and the target so that it scans a raster on the target. The beam is focussed magnetically by an axial field generated by a solenoid surrounding the envelope portion between the gun and the target. Deflection coils and focus coils are commonly assembled as a single focusdeflection coil assembly in a camera, and vidicon tubes are installed therein as required. The focus coil produces an axially directed field which acts on the radial component of velocity of diverging beam electrons to direct them in a helical motion back to the tube axis, where all the beam electrons converge to form a focussed spot. While most vidicons have a single electron gun with the gun axis coinciding with the magnetic focus field axis, a vidicon may have two or more guns spaced on opposite sides of the axis, each gun scanning a separate raster on the target. Such a tube may be used, for instance, for simultaneous pickup of two or more color component images for color transmission. Examples of multi-gun color pickup tubes are described in US. Pat. No. 2,294,820 issued Sept. 1,1942 to J. C. Wilson (Cl. 178-52) and US. Pat. No. 2,826,632 issued Mar. 11, 1958 to P. K. Weimer (Cl. l78-5.4).

A dual-gun vidicon having two off-axis guns and two separate target areas on the faceplate can be used particularly well to simultaneously transmit two separate color images when the two beams are deflected and focussed by a single focus-deflection coil assembly. One problem with such a vidicon, however, is that due to off-axis location of the guns, the beams are influenced in a nonuniform manner by the focus field. The field strength increases with the distance away from the axis.

Deflection of the beam thus exposes the beam to a varying field strength. When thegun is axially located, thenon-uniform focussing effect is largely symmetrical about the central axis and is relatively non-critical. With two off-axis guns, on the other hand, the nonuniformities are not axially symmetrical and are more pronounced, since the beams are farther from the axis in their trajectories than is a single axial gun beam. The result is non-symmetrical shading of the image output due to non-uniform landing of the beam on the target.

Non-symmetrical shading in the output images of a dual-gun vidicon is an especially serious problem for color pickup, since the strengths of the color signal must be precisely balanced to obtain faithful color reproduction. Non-uniformity in one or both color signals results in a spurious predominance of a color in a portion of the image. While the shading in one raster may be corrected somewhat by adjustment of an alignment current in the deflection coils, such adjustment only aggravates the shading in the other raster.

Non-symmetrical shading also degrades the registration of the two output signals. Such registration is criti cal for color pickup.

SUMMARY OF THE INVENTION In a novel magneticallyfocussed off-axis cathode-ray tube, each gun is oriented to the axis in such a way that it has a non-zero angle of tilt with respect to the axis and also a non-zero angle of skew.

With the gun oriented as described, the non-uniform effects of the magnetic focussing field on the off-axis beam are compensated so that non-symmetrical shading is essentially eliminated.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a partially broken-away, perspective view of a vidicon camera tube comprising a preferred embodiment of the invention.

FIG. 2 is an exaggerated perspective view of the faceplate and target of the tube of FIG. 1.

FIG. 3 is an exaggerated longitudinal sectional view of the electron gun assembly of the camera tube of FIG.

PREFERRED EMBODIMENT OF THE INVENTION A preferred embodiment of the invention is the color television vidicon camera tube 10 shown in FIG. 1. Beginning from the image input end. of the tube 10, the tube 10 has a transparent faceplate 12 about 38 mm (millimeters) in diameter, sealed with an indium seal l4to a bottle portion 16 of the envelope. On the. inside surface of the faceplate 12 is a photoconductive target 18, shown in greater detail in FIG. 2. The target 18 has a pair of aluminum film contact layers 20 which are connected to conductive glass frit buttons 22 sealed through the faceplate 12 to provide low capacitance signal output contacts at the outside surface of the faceplate 12. On the contact layers: 20 is a pair of semicircular, transparent, conductive tin oxide signal electrode layers 24 spaced from one another by 0.65 mm. The aluminum contact layers20 minimize the effects of the resistivity of the signal electrode layers 24. Coated uniformly over the signal electrode layers 24 and over the space between them, is an antimony trisulphide photoconductive layer 26. Referring back .to FIG. 1, the inside surface of the bottle portion 16 of the envelope is coated with a nickel electrode 28. In the end of the tube 10 opposite the target 18 there is an electron gun assembly 30 shown in greater detail in FIGS. 3 and 4. The assembly 30 has a pair of electron guns 32a and.32b opposite one another and spaced from the central axis of the tube 10.

Each gun 32 includes a cathode mount 34 having a thermionic electron emitting cathode surface 36, an aperture plate 38, an aperture cup 40 aligned with the aperture plate 38, and a second aperture cup 42 inserted and centered in the first aperture cup 4419. Spaced from the second aperture cup 42 is a focussing cylinder electrode 44 welded to a common plate 46. To the common plate 46 is attached an annular getter fixture 48, which also acts as a further focussing electrode, in addition to its getter function. The various electrodes of the guns 32 are connected to the tube stem leads 50, shown in FIG. l.

The axes of the guns 32a and 32b, shown in FIGS. 3 and 4 by the alternately dotted and dashed lines 52a and 52b respectively, are at an angle with respect to the principal axis of the tube 1@, shown by the alternately dotted and dashed line 54. Each gun axis is spaced about 6 mm from the principal axis of the tube 110 at the cathode surface 36. The guns 32 each scan a rectangular raster about 6.2 mm by 9.75 mm on the target 18 as shown by the dotted lines 56 in FIG. 1.

For clarity, the angle of the guns 32 withe respect to the principal axis of the tube 10 is described herein as the sum of the two component angles, a tilt angle 6 and a skew angle (1). The tilt angle 6* is a 4 degree tilt of the guns toward the tube axis in the direction of electron flow, and is shown greatly exaggerated in FIG. 3%. The skew angle d) is a 2 degree angle of the front of the guns counterclockwise, as viewed from the stem end of the tube 10. The skew angle do is shown greatly exaggerated in FIG. 4, which is a view of a section at 90 with respectto the section of FIG. 3.

FIG. illustrates the relationships between the offaxis field strength and direction of the electron beams from the guns 32 of the tube iii. The tube MD is shown in outline form, assembled with the focus-deflection coil assembly 58. The dashed projection line 69 is perpendicular to the tube axis and marks the approximate axial location of the gun cathode surfaces 36. The alternately dotted and dashed projection line 62 marks the approximate axial location of the target 18. The solid line curve 64 below the tube W and coil assembly 58 represents the normalized magnetic focus field strength, equal to the actual field strength divided by the maximum field strength, along the tube axis between the projection lines 60, 62. Below the line curve 64 is a dashed line curve 63 representing in exaggerated form the change in the magnitude of the lateral velocity component of an average beam electron in the tube as it travels from the cathode surface 36 to the target 18. The electron begins at the projection line 60 with an initial lateral velocity component and, after traveling through the focus field, strikes the target 18 orthogonally with zero lateral velocity component. This indicates how the tilt and skew angles 6), Q5 of the guns give the average beam electron an initial lateral velocity component in a direction such as to result in the desired orthogonal uniform beam landing on the target, thereby essentially eliminating shading.

GENERAL CONSTDERATIONS The invention is applicable to tubes having an offaxis electron beam which is magnetically focussed and is deflected. The magnetic focus need not be the only focus to which the beam is subject. The tube may have any number of electron beams off the axis. Magnetically focussed tubes with off-axis beams include such tubes as certain kinescopes, oscillographs, and camera tubes. Non-symmetrical shading is generally not a probelm except where the beam is deflected in a raster to pick up or to form an image. The principal axis of the tube is herein taken to be the central longitudinal axis of the magnetic focussing field which is applied to the tube for operation. For vidicons, this generally coincides with the geometrical principal axis of the cylindrical tube bottle portion. The gun axis is herein taken to mean a line along the general direction of the electron beam in the gun before it is significantly affected by the magnetic focus field.

The tilt and skew angle magnitudes are determined somewhat by the spacing distance of the guns from the principal axis. Since the magnetic focussing field is uniform only along the principal axis, the further the guns are off axis, the more compensation is needed to eliminate nonuniform beam landing. Therefore, the tilt and skew angles should be somewhat greater for a greater spacing of the gun from the principal axis. However, as a practical matter the tilt angle will be on the order of 2 to 6 and the skew angle will be on the order of 1 to 4 for vidicons having multiple guns off-axis, since the spacing of the electron guns is limited by the size of the guns generally used in such tubes and by the diameter of the envelope.

The shading due to the non-uniform beam landing becomes less objectionable as the scanned raster is made smaller. However, the size of the scanned raster in a vidicon tube is generally on the order of 6.2 millimeters by 9.75 millimeters for reasons of desired resolution and a practical tube size. Vidicon tube diameters vary somewhat, but are generally between 1.75 cm to 5 cm.

The invention has been described with the use of the term electron gun." This term, as used herein, is taken to refer in general to a means for producing an electron beam. The axis of this means is taken as along the average electron path as the electrons are just entering the strength of focus field which significantly affects their trajectory for beam landing purposes. Thus, an important feature of the present invention is that regardless of the means for generating the beam, the electrons enter the focus field with an average velocity at a tilt and skew angle with respect to the focus field axis as described herein.

I claim:

l. A magnetically-focussed cathode-ray tube of the type comprising:

an evacuated envelope having a principal axis along the axis of magnetic focus, and

an electron gun spaced from said principal axis and having a cathode and gun axis along the electron beam direction in said gun, wherein the improvement comprises:

said electron gun having an orientation with respect to said principal axis, said orientation comprised of a non-zero tilt angle and a non-zero skew angle with respect to said principal axis,

said tilt angle being an angle between said principal axis and an orthogonal projection of said gun axis in a plane intersecting said gun axis at said cathode and containing said principal axis,

said skew angle being an angle between said gun axis and said plane.

2. The tube defined in claim 1, wherein said tilt angle is from about 2 to about 6.

3. The tube defined in claim 1, wherein said skew angle is from about 1 to about 4.

7. The tube defined in claim 6 comprising a faceplate with a target adapted for two separate and adjacent rasters to be scanned on it by electron beams from said guns, said rasters being rectangular and being about 6.20 mm wide and about 9.75 mm long, said rasters being spaced from one-another by about 0.55 mm, said tilt angle being about 4 and said skew angle being about 2. 

1. A magnetically-focussed cathode-ray tube of the type comprising: an evacuated envelope having a principal axis along the axis of magnetic focus, and an electron gun spaced from said principal axis and having a cathode and gun axis along the electron beam direction in said gun, wherein the improvement comprises: said electron gun having an orientation with respect to said principal axis, said orientation comprised of a non-zero tilt angle and a non-zero skew angle with respect to said principal axis, said tilt angle being an angle between said principal axis and an orthogonal projection of said gun axis in a plane intersecting said gun axis at said cathode and containing said principal axis, said skew angle being an angle between said gun axis and said plane.
 2. The tube defined in claim 1, wherein said tilt angle is from about 2* to about 6*.
 3. The tube defined in claim 1, wherein said skew angle is from about 1* to about 4*.
 4. The tube defined in claim 3, wherein said tilt angle is from about 2* to about 6*.
 5. The tube defined in claim 4, wherein said gun cathode intersects said gun axis and is spaced about 6 millimeters from said principal axis.
 6. The tube defined in claim 5 wherein said tube is a vidicon television camera tube having two of said guns, said guns being spaced on opposite sides of said principal axis.
 7. The tube defined in claim 6 comprising a faceplate with a target adapted for two separate and adjacent rasters to be scanned on it by electron beams from said guns, said rasters being rectangular and being about 6.20 mm wide and about 9.75 mm long, said rasters being spaced from one-another by about 0.55 mm, said tilt angle being about 4* and said skew angle being about 2*. 